Pollen Feeding Reduces Predation of Northern Corn Rootworm Eggs (Coleoptera: Chrysomelidae, Diabrotica barberi) by a Soil-Dwelling Mite (Acari: Laelapidae: Stratiolaelaps scimitus)

Landscape diversification with flowering plants can benefit pollinators and natural enemies, although insect pests can also use floral resources for nutrition and chemoprotection. Corn rootworms (Coleoptera: Chrysomelidae, Diabrotica spp.) are major pests of corn (Zea mays L.), and while subterranean larvae primarily feed on corn roots, adult rootworms commonly consume floral resources from other plant species. We quantified the species, density, and sex of adult corn Diabroticite rootworm beetles on wild and cultivated sunflower, corn, and squash, quantified pollen within the bodies of adult northern corn rootworms [NCR, D. barberi (Smith & Lawrence)], and investigated how consumption of sunflower and corn pollen by NCR adults impacted predation of their eggs by two soil-dwelling mites with different feeding specialization. NCR were the most common Diabroticite species on sunflower inflorescences and western corn rootworm (WCR, D. v. virgifera LeConte) were more abundant in corn and squash blossoms. Pollen feeding by NCR adults did not impact egg predation by omnivorous Tyrophagus putrescentiae (Schrank) (Acari: Sarcoptiformes, Acaridae), but predatory Stratiolaelaps scimitus (Womersley) (Acari: Mesostigmata, Laelapidae) ate eggs less frequently and took longer to feed on eggs from NCR females that had fed on sunflower pollen. This research suggests pollen feeding by adult NCR can impact predation of their eggs. While increasing plant diversity can benefit natural enemies and pest control within agroecosystems, it is important to consider how floral resources alter dietary preferences of biocontrol agents.

Predation capacity of soil-dwelling predatory mites on two major maize pests

The western corn rootworm Diabrotica virgifera virgifera (WCR), and the wireworm Agriotes sordidus (WW), whose eggs and first instar larvae develop in the first few centimeters of soil, are major crop pests. As soil-dwelling predatory mites are known as potential biocontrol agents against many pests, we investigated the predation capacity of Stratiolaelaps scimitus, Gaeolaelaps aculeifer and Macrocheles robustulus on immature stages of WCR and WW in a laboratory setting. While eggs of WCR and WW were never consumed, all three predator species attacked both WCR and WW first instar larvae. While these results need to be confirmed in natural conditions, our work identifies the early larval stage instead of the egg stage as the most vulnerable stage for control against WCR and WW with soil-dwelling predatory mites.

Evaluation of Cosmolaelaps brevistilis and Stratiolaelaps scimitus (Mesostigmata: Laelapidae) as natural enemy of Bradysia aff. ocellaris (Diptera: Sciaridae)

Bradysia species are major insect pests of protected production systems, including strawberry crops in semi-hydroponic systems. In Brazil, its management is difficult due to the absence of available effective pesticides. The aim of this study was to evaluate the predatory efficiency of two species of mites (Stratiolaelaps scimitus and Cosmolaelaps brevistilis) under laboratory conditions. The experiments were conducted at 25 ± 1ºC, 75 ± 10% RH, in dark. In the first bioassay predation potential of each mite species, one female per arena, was evaluated on eggs, larvae (2–3 instar) and pupae of B. aff. ocellaris, replicated thirty times, and predation, oviposition, and survivorship were recorded for ten days. In the second bioassay, fly emergence was used to determine control efficacy. Each predator species was assessed at different densities (2, 5, 7 and 10) with ten larvae of B. aff. ocellaris in each experimental unit, replicated eight times. Stratiolaelaps scimitus performed better than C. brevistilis in both bioassays, however, on the eggs and pupae life stages, both species were not effective. On the larvae life stage, mean daily predation rate was 8.25 and 4.45 for S. scimitus and C. brevistilis, respectively. For each predator species no significant differences in fly emergence were found at the four densities, however at all densities S. scimitus provided better control than C. brevistilis. Further studies on potted plants are needed to determine the potential of these species for B. aff. ocellaris management.

Comparing the Life Table and Population Projection of Gaeolaelaps aculeifer and Stratiolaelaps scimitus (Acari: Laelapidae) Based on the Age-Stage, Two-Sex Life Table Theory

Predatory soil-dwelling mites, Gaeolaelaps aculeifer (Canestrini) and Stratiolaelaps scimitus (Womersley) (Mesostigmata: Laelapidae), are essential biocontrol agents of small soil arthropod pests. To understand the population characteristics of these two predatory mites, we investigated their development, survival, and fecundity under laboratory conditions. We used Tyrophagus putrescentiae (Schrank) as a food source and analyzed the data using the age-stage, two-sex life table. The duration from egg to adult for G. aculeifer was longer than that for S. scimitus, but larval duration was similar between the two species. Notably, G. aculeifer laid 74.88 eggs/female in 24.50 days, but S. scimitus laid 28.46 eggs/female in 19.1 days. Several population parameters, such as the intrinsic rate of increase, finite rate of increase, net reproductive rate, and gross reproductive rate of G. aculeifer, were significantly higher than those of S. scimitus. Using the bootstrap technique with 100,000 samples, we demonstrated that the life tables constructed based on the 2.5th and 97.5th percentiles of finite rate of increase (λ) net reproductive rate (R0) may characterize the variability in the survival and fecundity curves, as well as predict population growth uncertainty. These data provide important information for the practical application of predatory soil mites to manage agricultural pests.

A highly contiguous genome assembly of a polyphagous predatory mite Stratiolaelaps scimitus (Womersley) (Acari: Laelapidae)

As a polyphagous soil-dwelling predatory mite, Stratiolaelaps scimitus (Womersley) (Acari: Laelapidae), formerly known as Stratiolaelaps miles (Berlese), is native to the Northern hemisphere and preys on soil invertebrates, including fungus gnats, springtails, thrips nymphs, nematodes, and other species of mites. Already mass-produced and commercialized in North America and Europe, S. scimitus is now introduced in China as a biocontrol agent for field crop. The introduction, however, can lead to unexpected genetic changes within populations of biological control agents, which might decrease the efficacy of pest management or increase the risks to local environments. To better understand the genetic basis of its biology and behavior, we sequenced and assembled the draft genome of S. scimitus using the PacBio Sequel platform II. We generated ∼150 × (64.81 Gb) PacBio long reads with an average read length of 12.60 kb. Reads longer than 5 kb were assembled into contigs, resulting in the final assembly of 158 contigs with a N50 length of 7.66 Mb, and captured 93.1% of the BUSCO gene set (n = 1,066). We identified 16.39% (69.91 Mb) repetitive elements, 1,686 non-coding RNAs, and 13,305 protein-coding genes, which represented 95.8% BUSCO completeness. Combining analyses of genome family evolution and function enrichment of gene ontology and pathway, a total of 135 families experienced significant expansions, which were mainly involved in digestion, detoxification, immunity and venom. Major expansions of the detoxification enzymes, i.e., P450s and carboxylesterases, suggest a possible genetic mechanism underlying polyphagy and ecological adaptions. Our high-quality genome assembly and annotation provide new insights on the evolutionary biology, soil ecology and biological control for predaceous mites.